Contraction- Thrust Faults Earth 238-27

-contraction: (strain-related term). Shortening in a direction set by the acting field stress.

-accomodated by contractional fault.In the horizontal plane: thrust (shallow dip, less than30°) or reverse faults.

-Occur in zones of plate convergence

-Both subduction thrusts and « foreland thrusts » 

-Thrust systems especially well developed in collision zones

Chapters 17-18

Mechanics of Thrust Faults Earth 238-27

*What is the problem ?

Mechanics of Thrust Faults Earth 238-27

*Possible Solutions:

-Decrease Friction on Base:

-Gravity sliding

-Wedge Shape and Allowing Internal Deformation

Mechanics of Thrust Faults Earth 238-27

*Decrease Friction on Base:

f = C + n

f : Frictional Stress

C :Cohesion : Coefficient of Friction n: Normal Stress

sn may be decreased by increasing fluid pressure.The effect of an increase of the fluid pressure is to reduce the effective stress

n* = n - PH2O

Mechanics of Thrust Faults Earth 238-27

*Gravity sliding

Gliding modelno internal deformation

Mechanics of Thrust Faults Earth 238-27

Extrusion model-no deformation normal tothe direction of displacement -wedge deformed by pureshear

Spreading model-wedge spreads out radially-Internal deformation by non plane deformation

Mechanics of Thrust Faults Earth 238-27http://www-sst.unil.ch/research/seismic/W_Alps_main.htm

Mechanics of Thrust Faults Earth 238-27

http://www.geol-alp.com

Mechanics of Thrust Faults Earth 238-27

Mechanics of Thrust Faults Earth 238-27

Mechanics of Thrust Faults Earth 238-27

Mechanics of Thrust Faults Earth 238-27

*Wedge Shape and Allowing Internal Deformation

Mechanics of Thrust Faults Earth 238-27

Mechanics of Thrust Faults Earth 238-27

analogue modelling (buldozer model)

3D geometryEarth 238-27

Oblic Ramp

Front Ramp

Lateral Ramp

Contraction- Thrust Faults Earth 238-27

Two Major Types of Thrust Faults: Thin verses Thick-Skinned

•Thin -skinned deformation : -Basal or sole fault (decollement) of thrust system glides above basement. -Sedimentary cover is involved in faulting and basement slides beneath but is uncut by the thrusts. But since shortening must be taken up somewhere, basement is involved (unless the thrust system is due to a landslide).

•Thick-skinned deformation: basement and cover (wedge) are involved.-penetrative deformation (ductile)-formation of large thick nappes-extensive nappe-internal folding

Contraction- Thrust Faults Earth 238-27

Nappe system

Fold-thrust belt Earth 238-27

-A complex and well studied system, Thin-skinned thrust systems consist of a basal detachment or decollement, or sole thrust and more steeply dipping thrusts or imbricate thrusts (combined into imbricate fans) that branch off from the basal thrust.

-« in sequence » thrusting, duplex

Fold-thrust belt Earth 238-27

*Geodynamic setting

-Foreland of an Andean-type convergent margin

-Accretionnary prism bordering a trench

-foreland sides of a collisional orogenic belt

-Rift-inverted basins

-Seaward edge of passive-margin sedimentary basins

Fold-thrust belt Earth 238-27

Fold-thrust belt Earth 238-27

Seaward edge of passive-margin sedimentary basins

Fold-thrust beltEarth 238-27

*Balanced cross section

-Balance occurs when bed lengths and cross-sectional area are equal in both deformed and undeformed state (except with volume loss due to consolidation and dissolution

-Cross section must be able to be restored to undeformed state. That is, removal of deformation should result in group of undeformed sedimentary rocks with expectable geometry.

Fold-thrust beltEarth 238-27

Vercors balanced and restored cross-section (french Alps)

http://earth.leeds.ac.uk/tectonics/alpine_tectonics/index.htm

Fold-thrust beltEarth 238-27

Fold-thrust beltEarth 238-27

Fold-thrust beltEarth 238-27

*to sum up

-deformed-state cross section must be admissible

-Restoration of the cross section must yield reasonable geometries.

-Cross-section must be « area balanced »

-kinematiccaly reasonnable. You must provide step by step a model showing the evolution of the system from the undeformed initial stage to the final stage.

There can be several solutions for a balanced cross section.Acceptable solutions have to respect all these criteria